Transparent Co₃O₄/ZnO photovoltaic broadband photodetector

Abstract

Based on the requirements of transparent optoelectronic devices, herein we report a highly transparent broadband photodetector, which is based on spinel structured p-Co₃O₄ and n-ZnO heterojunction. The XRD and XPS analysis confirmed the wurtzite ZnO phase along with the formation of spinel structured Co₃O₄. The structural property shows the good crystalline properties of the heterostructure thin films. The optical properties reveal the dual absorption states of Co₃O₄ corresponding to the Co²⁺ state (E_g = 1.93 eV) and Co³⁺ state (E_g = 1.42 eV) along with wide band gap ZnO (3.2 eV). Moreover, the photodetector shows an impressive transmittance of ~76% in the visible window. Further, the photoelectrical performance shows the prominent photovoltaic effect (self-powered operation) with a significant photosensitivity of 4.57 × 10⁴. Moreover, the device is also capable of absorbing broad wavelengths from ultraviolet (UV) to infrared (IR) range. Importantly the device exhibits a fast response speed of 81.7 μs and a decay time of 178.8 μs which are found to be the fastest speed reported for transparent metal oxide-based p-n heterojunction devices under white light and self-powered mode. This work thus provides a comprehensive way of utilizing the robust metal oxide layers for developing a fast-high performing transparent self-powered device. The dual bandgap configuration of spinel Co₃O₄ demonstrates the wide range photo-responses, which is a promising feature for broadband transparent photodetectors and enhanced transparent photovoltaics.

Introduction

Energy harvesting from light sources in the form of simple photodetector geometry have reached a milestone in the past years [1,2]. Numerous configurations of high-performance photodetectors are based on external-bias mode of operation. The benefit becomes more effective when the photodetectors can be operated under self-biased mode along with broadband light absorption capability [[3], [4], [5]]. However, the fulfilment of this requirement is quite challenging which depends on various critical factors such as the choice of photoactive materials, device architectures and the intermediate supporting layers. In general, the planar heterojunction configuration is well adopted for the intended purpose with specifically chosen p-type and n-type materials [3]. Most metal oxide photodetectors are based on p-n heterojunction, which have limited range to photoresponse spectrum [[4], [5], [6], [7]]. Nevertheless, it is a pivotal task for the metal oxide-based photodetectors to respond the broad spectral range of the electromagnetic radiation, while maintaining the proper transparency towards visible light for wide photoelectric applications, such as transparent photovoltaics and photo-sensors.

A wide variety of heterojunction photodetectors are addressed in literatures [8]. Recently Quyang et al. have reported interesting BiOCl nanosheets/TiO₂ heterojunction arrays for ultraviolet photodetection [9]. A remarkable responsivity and detectivity of 41.94 A/W and 1.41 × 10¹⁴ Jones, respectively are achieved. Similarly, Tao et al. have recently reported Graphene/GaAs based heterostructure enabling highly sensitive visible/NIR photodetection based on self-powered operation [10]. In addition, Long et al. have highlighted the SnO₂/NiO heterojunction photodetector sensitive towards ultraviolet light and offers a detectivity limit of ~10¹³ Jones [11]. Moreover, the heterojunction formed with perovskite SrTiO₃ and CuS–ZnS nanocomposite also displayed appreciable performance under 390 nm illumination with a response speed of 0.7–94.6 ms [12]. It is worth to mention that, very limited reports are available on spinel Co₃O₄-based photodetection. However, there are several literatures which are addressed to ZnO based photodetectors. For instance, Zhang et al. have reported a transparent ultraviolet photodetector based on nanofibers of ZnO/NiO heterojunction. The photodetector exhibited excellent ultraviolet selectivity with self-powered operation enabling a responsivity of 0.415 mA/W [13]. Additionally, Ning et al. have reported the silver-doped ZnO nanofibers for realizing a sensitive ultraviolet photodetector with an on/off ratio of 10⁴ at zero bias condition [14]. Moreover, an ultrahigh responsivity of 9.7 mA/W is highlighted by a solar-blind self-powered photodetector based on ZnO/Ga₂O₃ heterostructure [15]. It is to be noted that in the mentioned photodetector arrays the response time is quite slow (in millisecond to seconds) with the limit for broad wavelength detection. Therefore, in order to cover the broad spectral range and for achieving fast switching response it is necessary to design photodetectors by considering the photoactive materials.

The non-toxicity and earth abundant materials are important advantages for wide utilization and application in the broad scientific fields. Wide bandgap metal oxides are known to be promising candidates for constructing robust photodetector geometries. Zinc oxide (ZnO) is a non-toxic and earth abundant versatile wide bandgap n-type semiconductor which has proven as an efficient ultraviolet absorber and applied in various superior performance photodetectors [[16], [17], [18]].

On the other hand, p-type semiconducting metal oxides are configured with ZnO are nickel oxide (NiO), cuprous oxide (Cu₂O), spinel cobalt oxide (Co₃O₄) and so on [[19], [20], [21], [22]]. Among these oxides, a relatively abundant material is spinel Co₃O₄ which has been extensively studied in various fields such as in gas sensing, supercapacitors, lithium ion batteries, photocatalysis, and optoelectronics [[22], [23], [24]]. In general, the spinel Co₃O₄ is a blended valence oxide of CoO and Co₂O₃ with high oxygen content. Along with this, the spinel Co₃O₄ being a p-type conducting material allows the presence of intrinsic acceptor states, which promotes the trapping of electrons thereby readily creating the charge carriers transport. Due to the indirect and dual bandgaps of Co₃O₄ of 2.2 eV and 1.6 eV, according to Co³⁺ and Co²⁺ states along with its spinel structure, it is believed that the feature of Co₃O₄ dual bandgaps would provide a solution for wide photo responses [24,25]. This will all-together benefit advantage for broadband photodetectors and high-performing transparent photovoltaics. Till date, various synthetic approaches have been reported for Co₃O₄, such as, conventional solution processing, spray pyrolysis methods and chemical vapour deposition techniques [[23], [24], [25], [26]]. Sputtering method can be a promising approach for quality Co₃O₄ layer for large-scale applications. By using reactive sputtering, pure and robust Co₃O₄ thin film can be achieved [23,27]. Notably, for photodetection application, the spinel Co₃O₄ must show uniform nanostructures, good crystallinity and strong optical absorption.

In the quest of achieving high performance broadband transparent photodetector, we report the demonstration of Co₃O₄/ZnO heterojunction, which shows the outstanding photodetection performances for broadband wavelength range with significantly enhanced performances. The structural property confirms the formation of spinel Co₃O₄ along with the formation of ZnO. Similarly, the optical absorption demonstrates the formation of dual bandgap of Co₃O₄ at 1.93 eV and 1.42 eV, according to the Co³⁺ and Co²⁺ states. We have also explored the self-powered operation of the p-Co₃O₄/n-ZnO photodetector under broadband illumination covering the ultraviolet to infrared region. The transparent device outcomes with fast response speed with rise time of 81.7μs and fall time of 178.8μs thereby maintaining a transparency of ~76% in the visible-near infrared region.